Gracilaria vermiculophylla, a filamentous macroalga native to the western Pacific, looks so much like its Atlantic native Gracilaria sister species that its invasion of Atlantic waters went largely undetected until molecular techniques revealed it was in fact a different species. It doesn’t go unnoticed any more: a recent study of macroalgal abundance and distribution in Hog Island Bay, VA, found that Gracilaria vermiculophylla was by far the most abundant macroalgae species in all seasons and in all locations and elevations sampled, comprising 74% of the total algal biomass. Interestingly, Gracilaria abundance was also correlated with species richness and abundance of other filamentous algae species, which were often attached to or entangled with the invader. Along transects from the mainland across the lagoon to an offshore barrier island, the highest macroalgal diversity and biomass was observed in mid-lagoon. While similar patterns in other studies often have been attributed to nutrient gradients, these authors believe that abundant mid-lagoon shell substrate conducive to algal attachment is partly responsible for the patterns observed here.

While Gracilaria biomass seemed to support greater macroalgal diversity by adding structural complexity to a relatively homogeneous soft-bottom system, the high Gracilaria biomass observed here had some negative consequences as well. The authors noted several summer anoxic events associated with accumulation and oxygen-depleting dieoffs of high (>100 g dry weight/m2) biomass of Gracilaria. These events may become more common if the species’ aggressive spread continues. Its already wide distribution, along with others’ lab experiments, suggests this species has a broad tolerance of temperature and light variability and desiccation stress.

Common Reed Too Common for Mummichogs: Invader Provides Poor Habitat

While many studies have documented various (mostly negative) impacts of the widespread invasive plant Phragmites australis (common reed) in the eastern U.S., until recently none had examined marsh habitat quality as a function of stage of Phragmites invasion. Characterizing marsh sites over a large geographic area (study sites were in New Jersey, Delaware, and Maryland) as “natural marsh,” “initial invasion,” “early invasion,” and “late invasion,” researchers recently compared the marshes’ physical characteristics, hydrology, and use by a resident marsh fish, the mummichog Fundulus heteroclitus. Consistent with what other studies have found, biomass and plant height increased with increasing invasion stage, while stem density declined. Increasing amounts of Phragmites were associated with drier marsh conditions: decreased flooding frequency, water depth, and percent of marsh covered with standing water. As stage of invasion increased, the abundance of mummichog decreased, likely due to drier conditions at the more highly invaded sites. This pattern was observed in all marshes, but the effect was less prominent in Delaware. A rarer species, Fundulus luciae, was also less abundant where Phragmites was more abundant.

Taken together, the results of this study indicate that Phragmites makes for relatively poor habitat for estuarine fish. While marshes in the earliest stage of invasion were found to support relatively healthy fish communities, the trick, nearly impossible for managers to achieve, is to halt the progression of invasion at the initial stage. Once invasion has begun, it is likely that the changes documented here will come to pass – the common reed will become all too common.

Do These Oysters Need Salt? Landings and Salinity in the Gulf of Mexico

More than half of the U.S. oyster harvest comes from the Gulf of Mexico, so it makes sense that steps should be considered to protect or enhance this valuable fishery and to evaluate the potential impacts of management actions on oyster yields carefully. Because oyster yields are highest at intermediate salinities (in the middle of their range of 10 – 30 ppt), managers occasionally consider projects to improve Gulf oyster landings by reducing saltwater intrusion, thereby decreasing salinities to this optimal range. These plans would theoretically work if the oysters in question live at the high end of their salinity range, but if they’re at the low end of the range more freshwater would only decrease yields. An analysis of more than 50 years of oyster landings and freshwater discharge data in five Gulf states revealed that oyster landings were at their lowest when nearby river discharges increased, and conversely, landings went up when discharge was down.

A proposed $99 million project in Louisiana aims to improve Gulf oyster landings by building a river diversion near New Orleans which would open a new or expanded Bonnet Carré Spillway, diverting Mississippi River water into Lake Pontchartrain and Mississippi Sound. Historical data on spillway diversions revealed that in 25 of the 28 times the spillway has been opened, oyster landings in the state of Mississippi were lower than in the other four states included in this study. Taken as a whole, these results suggest that Gulf oyster yields will decline, rather than be enhanced, if freshwater flow increases. The author concludes that the Bonnet Carré Spillway project cannot be justified, at least on the basis of improving oyster yields.

DOM Dynamics in Denmark

As recent research has shown, just because organic matter in aquatic systems is in the dissolved phase does not mean it is inert. In fact, because dissolved organic matter (DOM) is the largest reservoir for organic carbon in the aquatic environment, it may feed the processes that lead to eutrophication by absorbing light, fueling bacterial respiration, and carrying large quantities of carbon, nitrogen, and phosphorous to estuaries and coastal waters. For these reasons, a better understanding of DOM dynamics is important. In one Danish estuary, land use was found to have significant impacts on the quality and quantity of DOM loadings. Forested subbasins of the watershed contributed substantial DOM throughout the year, but especially during the rainy season. Subbasins characterized by agricultural land use contributed a smaller, more constant flow of DOM to the estuary throughout the year. While forested lands provide more DOM than those used for agriculture, the nutrients bound in this material may be less bioavailable than those from agricultural lands. Results of the study suggest that reducing the agricultural area of the watershed to about 50% of the watershed’s total area could lead to a substantial decrease in eutrophication risk to the estuary and adjacent coastal waters.

One technique used in this study holds promise for future monitoring. Measurements of the colored component of DOM were found to be closely correlated with dissolved organic carbon (DOC) concentrations, so quick, inexpensive measurements of DOM may be easy surrogates for DOC concentrations, at least in this Danish system. Because the DOM/DOC relationship can vary from site to site, it would have to be established in an individual system before this method could be used.